Are Qin and W equivalent in the Rankine and Otto cycles?

In summary, the experts have stated that the equation Qin = h3 - h2 is correct for the ideal Rankine cycle, and it is incorrect to subtract work from it. For the Otto cycle, Wnet in kJ/kg can be used to calculate power, but it is necessary to also know the mass flow rate to get the actual power. The work generated in the constant pressure process of 2->3 is counted in the enthalpy.
  • #1
physea
211
3
Hello!

In ideal Rankine cycle, we say that during the isobaric expansion, the heat provided to the system is Qin = h3 - h2
However, in my opinion, it is Qin - W = Δh -> Qin - PΔV = Δh. Which is correct?

Secondly,
In Otto cycle, let's say you have calculated Wnet in kJ/kg. In order to be able to calculate Power, isn't it necessary to know mass flow rate?

Thanks!
 
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  • #2
physea said:
Hello!

In ideal Rankine cycle, we say that during the isobaric expansion, the heat provided to the system is Qin = h3 - h2
However, in my opinion, it is Qin - W = Δh -> Qin - PΔV = Δh. Which is correct?

Secondly,
In Otto cycle, let's say you have calculated Wnet in kJ/kg. In order to be able to calculate Power, isn't it necessary to know mass flow rate?

Thanks!
The answer to the question about the Rankine cycle is that the first equation is correct (and yours is wrong).

The answer to the question about the Otto cycle is yes and no. It is acceptable to express the power as Wnet in kJ/kg if you also know that all you need to do to get the actual power is multiply by the mass flow rate.
 
  • #3
Chestermiller said:
The answer to the question about the Rankine cycle is that the first equation is correct (and yours is wrong).

The answer to the question about the Otto cycle is yes and no. It is acceptable to express the power as Wnet in kJ/kg if you also know that all you need to do to get the actual power is multiply by the mass flow rate.

Why my equation is wrong?
Isn't 2->3 a constant pressure process where the volume expands?
Isn't there work generated? Why we don't count it?
 
  • #4
physea said:
Why my equation is wrong?
Isn't 2->3 a constant pressure process where the volume expands?
Isn't there work generated? Why we don't count it?
It is counted. It’s lumped in with the enthalpy.
 

1. What are the Rankine and Otto cycles?

The Rankine and Otto cycles are thermodynamic cycles commonly used in power plants and internal combustion engines, respectively. The Rankine cycle is a theoretical cycle used to model steam power plants, while the Otto cycle is a theoretical cycle used to model gasoline engines.

2. Are the Rankine and Otto cycles equivalent?

No, the Rankine and Otto cycles are not equivalent. They have different thermodynamic processes, efficiency, and applications. The Rankine cycle involves the use of steam to generate power, while the Otto cycle involves the combustion of fuel to produce power.

3. What is the purpose of comparing Qin and W in these cycles?

The comparison of Qin (heat input) and W (work output) in the Rankine and Otto cycles helps to determine the efficiency of the cycles. It also allows for the optimization of the cycles to improve their performance.

4. How do Qin and W differ between the Rankine and Otto cycles?

In the Rankine cycle, Qin is the heat input required to convert liquid water into steam, while W is the work output produced by the turbine. In the Otto cycle, Qin is the heat released from the combustion of fuel, and W is the work output produced by the engine.

5. Can the Rankine and Otto cycles be compared directly?

Yes, it is possible to compare the Rankine and Otto cycles directly by using the same units for Qin and W. However, the comparison should take into account their different thermodynamic processes and applications.

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